Centrifugal pumps comprise rotating elements within a pump casing that increase the pressure of fluids traveling from an inlet port to a discharge port. One or more impellers are mounted on a shaft within the pump casing and increase the pressure on the fluid as the fluid travels through the casing. A motor drives the shaft to provide the rotating movement.
Some types of centrifugal pumps, which may be referred to as barrel pumps, use a plurality of impeller stages mounted on a common shaft. The impellers face the same direction and successively increase the pressure of process fluid as it flows through the stages.
Various forces act on the impeller shaft during operation of the pump. Forces acting along a direction parallel to the shaft's axis of rotation may be referred to as axial forces, and forces acting perpendicular to the axis may be referred to as radial forces. A long shaft must typically be radially supported at intermediate points along its length to prevent excessive sagging or curvature. Many pumps utilize bushings that closely fit around the shaft are mounted between the impellers to counteract radial forces and to maintain the desired radial position of the shaft.
Axial thrust also acts on the impellers during operation. The axial thrust is additive for each impeller, and very strong axial forces may develop along the shaft, depending on the number of stages. Each stage comprises an impeller (i.e. a rotating element coupled to be driven by the shaft) and a diffuser (i.e., a stationary element within the casing to promote smooth flow of fluid). All of the stages are housed in the casing. An axial force generated by the difference between the low pressure at the inlet of the pump and the high pressure at the outlet of the pump can also act on the shaft in the same direction as the axial thrust generated by each impeller. Axial forces in this direction may be referred to as active loads.
Pumps used in some systems, such as for example reverse osmosis systems, can also be subjected to axial force in the opposite direction, which is sometimes referred to as a “reverse load”. Such reverse load can occur, for example, when the pump is turned off, if the pressure downstream of the pump drops rapidly, or in other situations. For example, reverse loads can occur if a pump is started accidentally or improperly (e.g., with a discharge valve fully open), or if a check valve downstream from the pump in a reverse osmosis system is malfunctioning.
FIG. 1 shows an example of a basic desalination system 10 according to the prior art. Low pressure untreated water 11, which may for example be sea water, is provided to the inlet port of a high pressure pump 12. The pump 12 increases the pressure of the water to provide high pressure untreated water 13 to a reverse osmosis (RO) unit 14. In large scale desalination systems, the pressure at the outlet port of the pump 12 may be several hundred psi or more greater than the pressure at the inlet port of the pump, so as to overcome the osmotic pressure of salt in sea water and maximize the output of treated water. The RO unit 14 outputs treated water 15 with a relatively low salt concentration and condensate 16 with a relatively high salt concentration. During normal operation, the shaft of the pump 12 is subjected to an active load in the direction indicated by arrow 17. The shaft of the pump 12 may also be periodically subjected to a reverse load in the direction indicated by arrow 18. The pump 12 typically has a bearing assembly (not shown) for accommodating active loads, and a pressure bleed-off circuit 19 that provides the discharge pressure at or near the outlet to a balancing drum or disk closer to the inlet of the pump in order for accommodating reverse loads.
The condensate 16, which exits the RO unit 14 with a relatively high pressure, optionally passes through an energy recovery device (ERD) 21, and exits through a valve 22 as low pressure condensate 23. The ERD 21 uses the pressure of the high pressure condensate 16 to increase the pressure of a secondary stream of untreated water 11A, which exits the ERD 21 as high pressure untreated water 24, which joins with the high pressure untreated water 13 from the pump 12 and is provided to the RO unit 14. A circulation pump 25 may be provided to circulate the high pressure untreated water 24. Such a system may be more susceptible to reverse loading because of the pressure of the high pressure untreated water 24.
U.S. Pat. No. 6,309,174 discloses a thrust bearing for multistage centrifugal pumps that has a balance disk coupled to the pump shaft within a bearing cavity separated from a discharge chamber of the pump by a sidewall. Throttle ports are provided through the sidewall, which are configured to control the flow rate of process fluid therethough as the balance disk moves axially to allow pressure to build up and be relieved from a pressure cavity between the balance disk and the sidewall to control the axial position of the balance disk. Sealing lands on the sidewall are provided to take up axial loads not accommodated by the fluid pressure on the balance disk. Such a bearing has only limited capacity to take up reverse loads, which are only accommodated by the fluid pressure on the balance disk.
The inventor has determined a need for pumps with improved bearing assemblies.